In such reactors, thermal striping conditions, consisting of rapid temperature fluctuations of significant amplitudes, may occur in the liquid metal flow and this can give rise to difficulties if such temperature fields arise in close proximity to component structures.
More specifically, in a liquid metal-cooled fast breeder nuclear reactor, the fuel assembly comprises a multiplicity of slender fuel elements or pins over which liquid metal coolant such as sodium flows. The fuel assembly is divided into a plurality of sub-assemblies, each comprising a plurality of fuel pins enclosed by a tubular wrapper and having a lifting head. The sub-assemblies are positioned in side-by-side array and each one is located in cantilever manner by a lower and spike which is plugged into a fuel assembly supporting structure. The sub-assemblies in the central region of the fuel assembly mainly comprise fissile material whilst the sub-assemblies in the surrounding outer region comprise breeder material. During operation of the nuclear reactor the fertile material captures neutrons emitted by fission in the fissile material to produce further fissile material. As irradiation of the fuel assembly progresses and the fissile content of the breeder material increases, fission of some of the newly formed fissile material takes place so that the power output of the breeder sub-assemblies and, therefore, the coolant temperature, progressively increases. Varying temperatures of coolant streams flowing from the fuel assembly give rise to the condition known in the fast reactor art as thermal striping, a condition describing the production of rapid temperature fluctuations which can give rise to a risk of cracking in reactor structure material.